Optical coherence tomography (OCT) is a technique for obtaining high resolution cross-sectional images of tissues or materials, and enables real time visualization. The aim of the OCT techniques is to measure the time delay of light by using an interference optical system or interferometry, such as via Fourier Transform or Michelson interferometers. A light from a light source delivers and splits into a reference arm and a sample (or measurement) arm with a splitter (e.g., a beamsplitter). A reference beam is reflected from a reference mirror (partially reflecting or other reflecting element) in the reference arm while a sample beam is reflected or scattered from a sample in the sample arm. Both beams combine (or are recombined) at the splitter and generate interference patterns. The output of the interferometer is detected with one or more detectors, such as, but not limited to, photodiodes or multi-array cameras, in one or more devices, such as, but not limited to, a spectrometer (e.g., a Fourier Transform infrared spectrometer). The interference patterns are generated when the path length of the sample arm matches that of the reference arm to within the coherence length of the light source. By evaluating the output beam, a spectrum of an input radiation may be derived as a function of frequency. The frequency of the interference patterns corresponds to the distance between the sample arm and the reference arm. The higher frequencies are, the more the path length differences are.
Certain applications of OCT, such as multimodality OCT (MMOCT) or OCT/laser tissue coagulation, may make use of fibers with more than one clad in the sample arm and sometimes dual clad fiber couplers. When using double clad fibers (DCF), coherent detection suffers from crosstalk noise artifacts cause due to the use of DCF. Additionally, crosstalk between inner cladding modes and the core mode occurs at coupling interfaces causing image artifacts. Coupling interfaces include, but are not limited to, splices, connector to connector mates, and free space coupling from lens to lens or lens to fiber. The artifacts occur when inner cladding modes, excited at a primary crosstalk site, propagate within the inner cladding and couple back into the core at a secondary coupling site. Such issues may arise with other fiber configurations, such as, but not limited to, multi-clad fibers and fiber couplers.
To reduce crosstalk, current state of the art relies on two methods: (i) making fiber segments between coupling interfaces long enough such that crosstalk noise is deeper than the imaging range and cannot be seen; and (ii) attenuating the sample signal level such that crosstalk noise amplitude falls below a system noise floor and cannot be seen.
However, there are various issues related to these methods. For example, while making fiber segments between coupling interfaces longer in above method (i) may be manageable in a small subset of applications, this method may not be desirable for applications that, for example, require short probes or scanned probes. Making a probe unnecessarily longer can reduce its usability and increase its cost. Similarly for scanning probes like ones employed in luminal organ imaging like intravascular, lung, and GI tract the longer the probe the potentially worse the non-uniform rotational distortion (NURD) is and the substantially higher the background noise is. A higher background noise can make it harder to detect weak signals like Raman or auto-fluorescence from the sample.
By way of additional drawings of above method (ii), method (ii) relies on attenuating optical power to levels much lower than the maximum permissible power; limited mainly by a material's damage threshold or the maximum permissible exposure limit. Reducing power incident on the sample will reduce system sensitivity and therefore lower image penetration depth. This is usually undesirable since for most applications a goal is to maximize penetration depth in order to adequately visualize sub-surface structures deep within the sample. For weakly reflecting samples, this method may be detrimental for samples that have a combination of strong reflectors and weak reflectors since one would need to substantially reduce sample power level so as to have the crosstalk noise amplitude from strong reflectors fall below a system noise floor causing the signal from low reflecting structures to fall below the noise floor also or be barely visible. Strong reflectors may include stent struts embedded in the vessel wall, or air to sheath or lens to air interfaces in a catheter, endoscope, or capsule, among others. Weaker reflectors may include vessel wall subsurface structures like the media, intima, and adventitia in a coronary artery.
Accordingly, it would be desirable to provide at least one OCT technique and/or device for use in at least one optical device, assembly or system to avoid the aforementioned issues while mitigating crosstalk noise, especially in a way that reduces or minimizes cost of manufacture and maintenance.